1. Field of the Invention
The invention relates to a method and device for the application of a liquid medium onto a material web, and, more particularly, to a method and device for the application of liquid through viscid mediums onto a pre-dried material web.
2. Description of the Related Art
In the direct application process a liquid or viscid medium is applied directly by an applicator device to the surface of a moving material web, which is supported during the application process by a rotating support surface, such as a backing roll or a continuous belt. The liquid or viscid medium is initially applied to a carrier surface, such as the surface of a roll serving as an applicator roll, or the surface of one side of a flexible belt, and is transferred therefrom to the material web.
Indirect application is normally accomplished by a so-called film press implemented by two rolls, which together form a nip, and which transfer the medium successively or simultaneously to both sides of the material web or to only one side of the web.
Reference is made to U.S. Pat. No. 5,683,509 which discloses a flexible continuous belt, together with a transfer roll, which form the press nip through which the web travels. A press shoe is located on the inside of the continuous belt, thereby extending the nip and pressing the coating medium, that is applied by this unit, into the web. This improves the coating result, specifically by avoiding film splitting.
Reference is also made to DE 198 23 739 A1, according to which, a material web is coated in the wet section or immediately following the wet section, of a paper machine.
Film or size presses have been in operation for years. They have some significant disadvantages when utilized with today's high-speed machines, and depending upon the type of fiber web and coating medium, they do not always provide sufficient coating quality.
The raw material quality of paper or cardboard is continuously degrading. This is particularly true of the production of corrugated board base paper, which is largely manufactured from recovered paper. There is also an ever increasing demand for a lower mass per unit area (also referred to as basis weight). The result of using poor raw material quality and lower basis weight is that the tensile strength of the web, following the film press coating application, is very low, resulting in frequent web breaks after the coating of the web. This results in enormous production down times and associated high costs.
Film Presses, variously known as Speedsizer, Speedcoater, Optisizer or metering size press, frequently cause nip flattening and crushing in the nip. These effects are particularly negative in corrugated board production.
In the field, web breaks, particularly in the production of corrugated board base paper, are reduced by using modified starches, that have a low viscosity and a high solids content, as a coating medium. The low viscosity provides effective penetration and the high solids content produce low remoistening, thereby rendering possible only a low drop in tensile strength following the film press. However, modified starches are more expensive as compared to crystal starches.
Even these measures do not always lead to satisfactory results.